The present invention relates to rapid thermal annealing and more specifically to the use of a temporary overcoat applied to a substrate before annealing.
Rapid thermal processing (RTP) is known in the prior art as a way to anneal semiconductor wafers and other substrates in order to crystallize amorphous Si films and activate doped Si films. For example see, R. Kakkad, et al., "Crystallized Si Films by Low-temperature Rapid Thermal Annealing of Amorphous Silicon", J. Appl. Phys. 65 (5), 1 Mar. 1989. Rapid thermal annealing is a process of heating semiconductor devices quickly, where the anneal time is on the order of 5 seconds. Very rapid thermal processing (VRTP) is a process where the substrate surface is heated to 1000.degree. C. in less than 0.5 seconds. VRTP is also known in the art. For example see, "Rapid Thermal Processing: How Well Is It Doing and Where Is It Going?", Mat. Res. Soc. Symp. Proc. 92, 3 (1987).
One use of RTP and VRTP is solid phase recrystallization (SPC). Prior to SPC, a substrate will have on it one or more overlapping deposited silicon films which do not have a defined crystalline structure. Such silicon films are called amorphous silicon (a-Si). Heating an a-Si film to a sufficiently high temperature transforms it into a crystallized, or polysilicon, film.
Another use of RTP and VRTP is to integrate doped impurities into the crystal structure of a doped polysilicon film. The heat of RTP activates the impurities in the film, and increases the conductivity of the film. In a typical substrate processing operation, several films containing patterns of circuit elements are grown on or etched from the substrate, and then selected areas of the patterned films on the substrate are doped with impurities. The substrate is then heated and cooled, thus activating the doped regions.
Both a-Si films and polysilicon films can be heated effectively by exposing them to radiated light energy from a xenon arc lamp, however a-Si films generally absorb more energy in the range emitted by xenon lamps than do polysilicon films. The ability of these thin (250-2500 angstrom) films to absorb radiated energy is dependent on the thickness of the film, the amount of crystalline structure in the film, and the impurity content of the film. In general greater absorption, and therefore quicker heating, occurs in thicker films, which are more opaque to the radiated energy.
The temperature reached in an exposed film is not only a function of the absorbed energy, but also a function of the rate of heat loss by conduction to the structure underlying the film. This underlying structure is made up of the substrate and any previously processed layers. The term "layer" is used herein interchangeably with the term "film", however "layers" better describes a substrate where many films are laid one on top of the other. The rate of heat loss from a film is a function of the temperature difference between the film and its underlying structure, the heat capacity of the film, and the geometry of the boundary between the film and the underlying structure. As an example of the effect of geometry on heat loss, in a film containing an etched pattern of circuit elements, smaller features of the pattern will dissipate proportionally more heat to a cold substrate than a larger feature in a film of the same thickness. This is because a feature can dissipate heat into parts of the structure lying beyond the edges of the feature as well as into the structure directly under the feature, and a smaller structure has a larger edge-to-area ratio, giving it a higher heat transfer coupling to the underlying structure.
Heat conductivity is also a function of the heat capacity of the underlying structure. Thus, if a film to be annealed is overlying a film which acts as a thermal insulator, the film being annealed will cool slower than if the film is in direct contact with a thermally conductive substrate.
These unavoidable variations in radiant energy absorption and heat dissipation lead to a common problem associated with annealing a patterned substrate, namely uneven heating of an uneven film surface. Larger features, as compared with smaller features, will absorb proportionally more radiated energy, since the cross section exposed to the xenon lamp is greater, and larger features conduct heat less efficiently to the surface of the substrate. Thus, larger features tend to overheat as smaller features are heated to annealing temperatures. The overheating problem also arises when trying to uniformly heat thick, multilayer features. A thin feature is exposed to as much radiated energy as a thick feature of the same area, consequently a thick feature will heat faster due to increased opacity and less thermal coupling per unit mass to the underlying structure.
Another disadvantage of radiant heating, in addition to the inability to create uniform temperatures, is that a film to be annealed must be uppermost on the substrate, where it is able to absorb the radiant energy. To meet this requirement, many annealing steps must occur in the production process, each before the layer to be annealed is covered by other layers. For example, in a typical thin film transistor (TFT) process, SPC would occur in the early stages of the process, when the layer to be recrystallized is uppermost, and implant activation would occur in the later stages of the process, when the implanted layer is uppermost.
Radiant heating of thin film structures on glass substrates presents additional difficulties. Typical substrate glasses cannot withstand extended exposure to temperatures above the glass strain point (usually in the range of 575.degree. C. to 650.degree. C.). Because crystallization of a-Si films and implant activation by RTP or VRTP may require heat treatment above the glass strain point, damage to the glass substrate may occur because exposure times are too long at the power density levels typically available to RTP/VRTP (10-5000 W/cm.sup.2).
To solve the problem of uneven temperatures, the substrate can be annealed in a convection furnace. However, this method also has its drawbacks. Furnace heating takes longer, and as 8 inch wafers become more common over 4 inch wafers, end-wafer heating problems will become more problematic. End-substrate heating problems also arise where the substrate is a glass substrate for a flat panel display, which can measure 8 inches across for a single device. Also, since the entire furnace chamber must be heated to the annealing temperature, the walls of the chamber are more likely to give off contaminants.
The substrate can be heated using a pinpoint laser scanning in two dimensions and adjusting the intensity of the beam to compensate for the variations in energy absorption and heat dissipation. However, for this approach to work properly, the intensity control of the laser must be closely aligned with the features on the substrate, which becomes increasingly complicated as feature size decreases. Laser annealing is also undesirable because of the high power densities required and the tendency for the large temperature gradients caused by spot scanning to damage substrates.